The present invention relates to liquid ink printers, and more particularly to a variable throughput active-passive dryer assembly for use in such a printer to handle wet printed sheets and stack them without ink offset and image smearing problems.
Printing in ink jet printers demands that excess moisture (generally water) on the surface of printed sheets be removed within a set time period and before the sheets are stacked. If the sheets are stacked before the images are dry, image smearing and offset occur. Devices that actively remove moisture, specifically water, from the sheet surface are referred to as dryer assemblies. A commonly used dryer assembly is a hot air convective mass transfer drying system. While dryer assemblies are effective in rapidly removing the excess moisture from the sheets, dryer assemblies greatly increase printer power requirements and size. If dryer assemblies were used in small printers, the size and cost of these printers would greatly increase. Also, most small printers do not have the power throughput to accommodate an active dyer.
The following references may be relevant as background for the present invention. U.S. Pat. No. 4,088,314 issued to Ronald Alfred Phillips, is directed to sheet handling in general, and discloses a synchronous document stacking device that includes a rotatable carrier member having spiral pockets for receiving documents at a loading position, and releasing them at an unload stacking position. Similarly, U.S. Pat. No. 4,385,756 issued to Jack Heery, is directed to sheet handling in general, and discloses a inverting and stacking apparatus that includes a rotatable carrier member having parallel arms and slots for receiving sheets at a loading position, and releasing them at an unload stacking position. The use of "Tined stacker wheels" or stacking devices of the rotating spiral carrier type as here, for inverting and or stacking sheets of paper, are well known. The present invention however incorporates and combines aspects of such a device to form a uniquely controllable mechanism for wet sheet drying in a liquid ink printer applications.
Directed to drying of wet sheets in a liquid ink printer, there is for example, U.S. Pat. No. 2,306,607 to Horton that discloses a web drying device for sheets printed by an intaglio printing apparatus. The web drying device feeds the web along a series of rollers while exposing the web to heat blown onto the web by an exhaust fan. The prior art references all teach the use of some type of active heating element to dry the freshly printed sheets. As discussed above, these active dryer assemblies demand increased printer power throughput and also increase the size and cost of the printing apparatus which is unacceptable for a small, relatively inexpensive printer.
U.S. Pat. No. 4,970,528 to Beaufort et al. discloses an ink jet printing apparatus having a uniform heat flux dryer assembly system which uses an infrared bulb and reflectors to transmit heat to the printed paper during the ink drying process. The freshly printed sheet is dried as it is fed from the printing apparatus along a 180 degree arc which surrounds the infrared bulb and reflectors.
U.S. Pat. No. 4,982,207 to Tunmore et al. discloses an ink jet printer having a rotary platen with a heater contained therein. The platen is made of a heat conducting material to transmit the heat from the heater unit enclosed therein to the outer surface of the platen to dry the printed sheet before it is fed to a stacking unit.
Thus, these small printers must rely on passive or low power drying systems to dry the wet sheets in order to maintain their low cost. However, most passive drying systems require large areas to avoid wet sheets from contacting any surface which will cause smearing and therefore, greatly increase the size of the printer. Additionally, due to the nature of some images being printed, an amount of ink deposited on some sheets is too much to be effectively dried passively before such sheet is placed on a stack.
Presently, most low end ink jet printers use fast dry ink formulations to allow low cost solutions to eliminate ink offsetting after printing. Ink offsetting is the unwanted transfer of ink from one printed page to another. These printers typically use holding stations that keep the presently printing page from contact with the previous one. Some such printers use a one sheet buffer to do this. For example, the one sheet is moved momentarily onto a set of output rails after being printed, and then it is allowed to drop onto an output stack. This approach allows the one sheet to dry undisturbed while the next sheet is being printed. However, more costly drying solutions are required when slower dry inks are used, or in higher speed higher throughput printers.
Some ink jet printers use preheating of the paper as well as high power active radiant heating in attempts to intercolor bleeding, and to aide in drying wet sheets. However, as liquid ink or ink jet printing devices become faster in speed, and require better print quality, better solutions will be required for wet sheets that move to fast to sufficiently dry before stacking, and thus usually susceptible to ink offsetting and smearing.
There is therefore a need for a low power, combination passive and active sheet handler and dryer assembly for use in liquid ink printers to effectively handle and dry wet sheets.